1. Field of the Invention
The present invention relates generally to multilayer film technology and more specifically to semiconductor technology. Most particularly, the present invention relates to the controlled positioning of a compound layer, such as a silicide layer, in relation to a semiconductor substrate, such as silicon. Such layers are typically used in fabricating complementary metal-oxide semiconductors (CMOS) transistors.
2.Background of Prior Art
It is well known by those skilled in the art that the electrical contact between a compound layer or film, such as a silicide and the silicon substrate in a semiconductor device is critical for good conductivity. Therefore, better contact between the compound layer and the substrate is always desired.
Problems with this contact region have been found when the semiconductor substrate contains various dopants which are generally added to improve the conductivity of the substrate. Contact metallurgists have discovered that when attempting to create a titanium silicide (TiSi.sub.2) compound layer adjacent to and in contact with the semiconductor substrate, the semiconductor substrate frequently deteriorates because of the required high temperature annealing process used to convert the titanium silicide to a more conductive crystalline form, such as C-54 titanium silicide. The high temperature anneal often causes the dopants already present in the semiconductor substrate to redistribute and, further, creates a roughening of the contact region between the doped semiconductor and the adjacent silicide layer. This roughening can cause the device to short circuit.
Another difficulty in positioning titanium directly on a silicon semiconductor substrate is found with the complication in forming a uniformly thin titanium silicide layer in close proximity to device junctions. Silicide thicknesses of less than 50 nanometers must be carefully positioned very close to the device junctions to prevent leakage or shorting. The precise positioning of such a thin layer is often difficult.
Yet another shortcoming with the known technology occurs because the doped silicon frequently chosen for the semiconductor substrate layer is much less reactive with titanium than undoped silicon making the formation of a titanium silicide layer more difficult with doped silicon. Thus, it is more preferable to grow a titanium silicide layer on a layer of pure or lightly doped silicon than directly on the doped silicon.
For these reasons, in some devices, a selective silicon epitaxial layer is grown adjacent to the semiconductor doped silicon layer. Since this epitaxial silicon layer, commonly referred to as epi-silicon, is usually undoped or only lightly doped, it reacts better with the titanium to form a titanium silicide compound surface layer. However, after reacting the titanium with the epi-silicon layer, portions of the epi-silicon layer usually remain since it is difficult to control the exact thickness of the epi-silicon needed to react with the metal. Because of its undoped form, the remaining epi-silicon layer generally degrades the desired contact between the titanium silicide and the underlying semiconductor substrate. This contact resistance caused by the epi-silicon layer renders the semiconductor device less conductive.
The present invention provides for a method of removing the intermediate epitaxial silicon layer without destruction or disturbance to the newly formed compound surface layer. Removal of the epi-silicon layer also positions the surface layer immediately adjacent to the silicon semiconductor substrate, thus further improving the contact region.
The present invention essentially provides for a controlled method to reduce the thickness of the intermediate epitaxial silicon layer by selectively removing atoms which are of the same type as those atoms comprising the epitaxial layer from the surface of the compound silicide layer while simultaneously heating the multilayer device, such that the diffusion differential between the compound atoms provides for increased mobility of one of the two atoms relative to the other. Because of this diffusion differential, atoms from the intermediate epitaxial silicon layer replace the selectively removed silicon atoms until all atoms of the epitaxial silicon layer have effectively replaced the removed atoms and are incorporated into the compound molecules. The benefit of the inventive method is that the elimination of the intermediate layer causes the positioning of the compound surface layer to change while its structure remains essentially unchanged.
Arima, et al., U.S. Pat. No. 4,983,547, teaches the formation of a silicide film from a deposited film which contains a higher than stoichiometric concentration of silicon (silicon rich). A film of aluminum or aluminum alloy is deposited on the silicon rich film, followed by heat treatment to precipitate the excess silicon into the aluminum film, leaving a stoichiometric silicide. However, the reference fails to teach how to eliminate one layer while leaving an adjacent layer effectively or structurally intact.
Gartner, et al., U.S. Pat. No. 4,248,688, discloses the use of ion beam etching to selectively remove platinum or palladium in the presence of their silicides. The process relies on high sputter etching ratios between the metals and the silicides. However, the reference fails to introduce any possibility of moving the silicide layer closer to or relative to another layer, such as a silicon semiconductor substrate, nor is there a reference with regard to removing an intermediate layer through the selective removal process.
The diffusion of one constituent toward the surface has been shown to cause preferential sputtering in alloys of Ni--Ag and Cu--Ni and in compounds PtSi, MoSi.sub.2 and NiSi requiring corrections to depth-profiling measurements of the composition. See for example J. Fine, T. D. Andreadis and F. Davarya, Nucl. Instr. and Methods 209/210 (1983) 521-530; M. Shikata and R. Shimizu, Surf. Sci., 97 L363 (1980); and Th. Wirth, V. Atzrodt and H. Lange, phys. stat. sol. (a) 82, 459 (1984). In these cases, diffusion during ion etching establishes composition gradients up to several micrometers deep determined by temperature and preferential sputtering yields. Bilateral Ni/Ni.sub.3 C has also been shown to develop highly preferential sputtering. K. Morita, H. Ohno, M. Hayashibara and N. Itoh, Nucl. Instr. and Methods in Phys. Res. B2 (1984) 596-600.
In general, there has been no teaching in the prior art of combining a method of mobilizing one of the types of atoms in a compound layer relative to the other and selectively removing atoms from the compound layer in order to eliminate an adjacent intermediate layer comprised of the same type of atoms as are being selectively removed from the adjacent layer, whereby the surface compound layer becomes positioned closer to a base layer through the controlled elimination of the intermediate layer, while the structure of the compound layer remains essentially unchanged.